1. Field of the Invention
The present invention relates generally to boiling water reactors and, more particularly, to methods and apparatus for repairing a cracked jet pump riser in a boiling water reactor.
2. Discussion of the Related Art
A typical boiling water nuclear reactor 10, as illustrated in FIG. 1, includes a reactor vessel 12, a core 14 made up of a plurality of fuel assemblies 16, and a core shroud 18 disposed concentrically within the reactor vessel around the core. Core shroud 18 includes upper and lower cylindrical sections 20 and 22 connected by a horizontal plate or ring 24 extending radially inward from a bottom edge of the upper cylindrical section to a top edge of the lower cylindrical section. A shroud head flange is welded to the top edge of the upper cylindrical shroud section and extends radially inward to support a shroud head or lid 26 of generally hemispherical or dome-shaped configuration, the lid being secured to the top of the shroud with bolts threadedly or otherwise engaged by lugs mounted in angularly spaced relation about the shroud periphery adjacent the top edge of the shroud.
Fuel assemblies 16 are supported at the bottom by a core plate 28 mounted on a core plate support ring or ledge 29, best shown in FIG. 4, extending radially inward from the bottom edge of the lower cylindrical shroud section and at the top by a top guide 30 mounted on horizontal plate 24. Control rod guide tubes 32 are provided within vessel 12 at locations above a control rod driving mechanism extending through nozzles located at the bottom of the reactor vessel beneath the shroud. Lower ends of corresponding control rods are detachably connected to the driving mechanism and are arranged to move up and down within the guide tubes.
Feedwater enters the reactor vessel via a feedwater inlet 34. The feedwater is distributed circumferentially within the reactor vessel by a ring-shaped pipe 36, known as a feedwater sparger, disposed above the shroud and having suitable apertures for circumferentially distributing the feedwater inside the reactor vessel. The feedwater mixes with other water coming from steam separators of the reactor and flows downwardly from feedwater sparger 36 through the downcomer annulus 38, that is, the annular region or space between the reactor vessel and the core shroud, and enters the core lower plenum 40. Boiling is produced in the core creating a mixture of water and steam which enters the core upper plenum, that is, the space under the shroud lid, and is directed into steam plenum heads or stand pipes 46 mounted vertically on the shroud lid in fluid communication with the core upper plenum. The mixture of water and steam flows through stand pipes 46 and enters a respective plurality of steam separators 48, which are shown as being of the axial-flow centrifugal type. The separated liquid water then mixes with incoming feedwater and flows downwardly to the core via the downcomer annulus. The steam, on the other hand, passes through a steam drying assembly or dryer 50 disposed above the steam separators and is withdrawn from the reactor vessel via a steam outlet 52.
Boiling water reactors also include a coolant recirculation system providing the necessary forced convection flow through the core. A portion of the water flowing through the downcomer annulus is withdrawn from the reactor vessel via a recirculation water outlet 42 and is fed under pressure into a plurality of jet pump assemblies 44 distributed circumferentially about the exterior of the core shroud to produce a forced convection flow through the core, thusly providing the required reactor core water flow. Boiling water reactors typically include between six and twelve jet pump assemblies; however, most reactors include ten jet pump assemblies. Referring to FIG. 2, it can be seen that jet pump assemblies 44 are distributed circumferentially about core shroud 18 in annular space 38 between the core shroud and reactor vessel 12. Each jet pump assembly 44 is positioned adjacent a recirculation inlet nozzle 54 formed on the exterior of the reactor vessel and, as best seen in FIGS. 3 and 4, the jet pump assemblies each include a riser assembly 56 extending upwardly from the recirculation inlet nozzle to a transition piece 58, and two inlet mixers 60 extending downwardly from the transition piece to a pair of diffusers 62 mounted over holes (shown in FIG. 1 at 64) in a pump deck 66 connecting a bottom portion of the shroud with the reactor vessel. Riser assembly 56 typically includes a riser pipe 68 oriented vertically in parallel relation to shroud 18 and a riser elbow 70 extending downwardly from the bottom of the riser pipe and bending outwardly to a circumferential weld 72 connecting the elbow with a thermal sleeve 74 in the recirculation inlet nozzle. The riser assembly is supported near the top by a riser brace 76, which is welded to riser pipe 68 and to pads (not shown) on reactor vessel 12 and/or shroud 18. Transition piece 58 extends in opposite directions from the top of riser pipe 68 to connect with inlet mixers 60 on opposite sides of the riser pipe. Inlet mixers 60 extend downwardly from transition piece 58 in parallel relation to riser pipe 68, with lateral support for the inlet mixers being provided by jet pump restrainer brackets 78 attached between respective lower ends of the jet pump inlet mixers and the riser pipe. The entrance or upper end of each inlet mixer 60 is clamped to the riser transition piece 58 by a beam-bolt assembly. The exit or lower end of each inlet mixer 60 forms a slip joint with the entrance or upper end of one of the diffusers 62, the interface between the inlet mixers and the diffusers providing additional lateral support for the riser assembly. The top of the slip joint is located near the bottom of the fuel assemblies; the exact elevation of the slip joint being dependent upon the particular boiling water reactor in question. Diffusers 62 extend downwardly from inlet mixers 60 to pump deck 66 and are of increasing diameter in the downward direction.
The riser elbow is typically a 10-inch diameter, 90 degree radius elbow fabricated of Type 304 stainless steel which, after periods of use, is susceptible to cracking along welded joints. Cracking is particularly common in the heat-affected zone of the weld joining the riser elbow with the thermal sleeve since this weld is typically made in the field and not in a shop where conditions are less likely to result in cracking. Separation of the jet pump riser piping in this area could adversely impact safety in some boiling water reactors under certain accident conditions. When more than one jet pump assembly is affected, the jet pump piping must either be replaced or repaired. Repair is certainly the preferred alternative in view of the fact that replacement involves significant expense, relatively long shut-down time, and the potential for radiation exposure to personnel.
Since weld repairs in the downcomer annulus are typically not practical due to inaccessibility, helium cracking, and the potential for excessive radiation exposure to personnel, a need exists for a method of repairing cracked jet pump riser assemblies involving little or no in-vessel welding.